The present invention relates to carburetor control systems for internal combustion engines and in particular to override speed control systems for small internal combustion engines such as those used with lawnmowers and other lawn and garden implements.
It is desirable that the speed of internal combustion engines be controlled so that the speed of the engine remains relatively constant under different loading conditions. For instance, in the use of a lawnmower which is powered by an internal combustion engine, it is desirable that the selected speed of the engine remain relatively constant despite various loading conditions. Thus, as the lawnmower encounters tall grass which loads the engine down or relatively short grass under which condition the engine has relatively low loading, the lawnmower speed which has been selected by the operator should remain substantially constant.
In the conventional design of small internal combustion engines, speed controls have been provided which control the carburetor to provide constant engine speed for different loading conditions. Such carburetors are normally provided with a choke valve and a throttle valve and have a governor connected with the speed control lever and the throttle and choke valves for maintaining constant speed. The governor conventionally comprises a pair of rotating flyweights which is rotated by the engine and has an output shaft or lever which is connected with the speed control lever and a throttle valve to maintain constant engine speed.
The speed control lever commonly consists of two parts, one of which is connected by means of a control cable to a manually operable control lever and the other of which is connected to the governor lever. The two parts cooperate to control the choke and throttle valves and are generally interconnected with an adjustment screw by which the high speed setting of the engine is adjusted. The control lever has an idle position, a start/run position and a choke position. The control lever is connected to the choke valve so that in the choke position the air flow to the engine is reduced and a rich fuel mixture is admitted to the engine.
By means of such type of controls, the speed of the engine will remain relatively constant for any of the settings of the control lever because of the interaction of the governor with the throttle valve and the control lever.
A particular prior art override speed control is disclosed in U.S. Pat. No. 4,517,942, which patent is expressly incorporated herein by reference. The override speed control disclosed in said patent comprises a speed control lever, an intermediate lever and a throttle control lever. The three levers are interconnected by means of extension springs, one each of which connects the intermediate lever respectively to the speed control lever and the governor lever. The speed control lever in the last increment of movement of its control range actuates the choke valve of the carburetor and the governor lever controls the throttle valve of the carburetor. A governor driven by the engine and responsive to engine speed is connected to the governor lever and adjustable stop means are provided on the intermediate lever to positively stop the movement of the intermediate lever during the last increment of movement of the speed control lever. During this last increment of movement of the control lever, the intermediate lever is unresponsive to the movement of the speed control lever, and the speed of the engine will be maintained at the high speed setting during movement of the speed control lever into the choke range.
A problem with certain prior override speed controls is that when the control lever is moved from the high speed range into the choke range, the tension increases on the control cable. This results in higher operating forces and gives the operator the feel of a stiffer control when moving into the choke range.